Method of pulping hardwood by a neutral sulfite process



Oct. 10, 1961 K. E. OLSON 3,003,909

METHOD OF PULPING HARDWOOD BY A NEUTRAL SULFITE PROCESS Filed Jan. 50, 1957 NELTED SULFUR Talus INVENTOR 29 K 2mm E. OLSON BY Kl-M and ATTORNEY United States atent C) 3,003,909 METHOD OF PULPING HARDWOOD BY A r NEUTRAL SULFITE PROCESS herm t E. Olson, Tomahawk, Wis., assignor, by mesne assignments, to Owens-Illinois Glass Company, Toledo,

Ohio, a corporation of Ohio iled Ja 195 1 N09 3 e (c 16 -3 hi ia ent e el tes to the a u a t P r- More'particularly this invention relates to a semi-chemical process incorporating a continuous chemical recovery system." More specifically this invention is concerned with the so-called neutral sulphite process utilizing an alkaline metal sulphite, e.g. sodium sulphite, as the pulpin; agent and'an alkaine metal carbonate, e.g. sodium carbonate, as the bufferingagen t.

The neutral sulphite pulping process, of course, finds particular application in the production of pulp from the hardwoods which are rich in liemicellulose'. Species of this hardwood are, for example, the poplar, oak, maple, gum, walnut, aspen, etc. H

The process with which this invention is concerned, and which will be discussed more fully hereinafter, comprises generally the reduction of logs of wood as supplied to the mill to a consistencyof fairly small chips, combining these chips with a hot aqueous solution, commonly referred to as a liquor, of the chemicals concerned, and cooking the thus combined chips and liquor under a specified steam pressure whereby the pulping agent component of the liquor attacks" the lignins 'and'other organic materials in the Wood releasing the cellulose for washing, filtering, refining and making into the paper.

Some of the chemicals constituting the original liquor are lost in the washing and filtering step, however, some of these chemicals are removed in the filtration operation as filtrate and from an economic point of view it is most desirable that the chemicals therein be recovered for reuse.

In general the recovery ofthe chemicals involves stripping off excess. water, burning. the remainder or residue, dissolving the non-combustible or smelt in water and adding makeup chemicals, namely, sodium carbonate and sodium sulphite to a prescribed digesting consistency necessary to eflicienctlyand economically accomplish the digestion of the wood chips when, reintroduced into the pulping process,

Heretofore neutral sulphite liquors for pulping have conventionally utilized a mol ratio of sodium carbonate to sodium sulphite, of about 1 to 7. Such a ratio has been reported as desirable in the making of unbleachable or coarse pulps giving a yield of about 80%. It has also been suggested that a ratio of l to 4 of carbonate to sulphite may be used in making bleachable pulps giving a yield of about 70%. For large scale operations, of course, it has been found desirable to utilize elemental sulphur as a source of the sodium sulphite pulping agent. This has been accomplished by converting the sulphur to sulphur dioxide gas by a burning operation and allow ing the sulphur dioxide gas to be absorbed into the soluiion of smelt products from the furnace yielding thereby the sodium sulphite. Even with this economical practice it is found that the sulphur burner and the subsequent absorption of sulphur dioxide involves some losses of the sulphur and, ofcourse, this is economically disadvantageous.

It has now been found quite unexpectedly that the amount of sodium sulphite may be substantially reduced without any apparent effect at all on the cooking time and the digesting operation and without any adverse effects on the final paper produced by the process. Thus for one mol of alkali bufier, expressed as soduim car- 3,003,909 Patented Oct. 10, 1961 'ice 2 bonate, it has been found that 2.5 or less mols of sodium sulphite may be conveniently used.

Exactly why the ratios of this invention achieve as efli eient pulping as was observed in the case of the ratios used heretofore is not known. However, it is obvious that with the lower amount of sodium sulphite required that the dependent requirements of elemental sulphur nd h atte da sse o su phur e su s n i lly reduced.

I heref re. an object of the present invention to provide a process for pulping which is continuous in n e. inco p r tes efficient r y ystem d re.- quires less makeup chemicals than required heretofore.

It is a particular object of the invention to provide such a process which is capable of producing paper equal in quality to that kuowrr'heretofore with substantial savings, in chemical costs.

These and other objects of this invention will become. apparent from the following detailed description of the mvenfiqn t k n in cai aet a h. the nn x d. hee of drawings on which is presented, for purposes of illustration only, a preferred embodiment of the invention.

The drawing is a schematic flow sheet of the steps in; volved in the, pulping operation and including a schematic diagram of the recovery and recycling of the chemicals.

Basically this invention involves the, discovery that the at f; a ka ne bu fer to. kal metal p i my be adjusted as discussed more fully hereafter effecting substantial savings in cost without any adverse, eifect in the overall operation of the process.

Referring how more specifically to the drawing there. is shown in a supply of pulpwood 11 usually in the form of 100. inch lengths. Thefwood, as supplied, contains about 40% water and about 60% solid, as determined by an oven baking test in which a weighted sample of the wood as supplied is kept; in a 240 F. oven for about 18 hours after} which it is reweighted." The lengths are slashed at 12 into two. 50 inch lengths and are subsequently barked 13, chipped 14, and conveyed by screws 15 to a silo 16, from. the silo the chips are fed at a rate of about 50,000, lbs. per hour into a continuous digester 17.

The digester or reaction chambers 17 consists of six 24" diameter 20 ft. long pipes (not shown) which are horizontally disposed and stacked in vertical array. Each of the-pipes contains a conveyor screw which pro,- pels the. chips longitudinally along the 20 ft. length, Alternate ends of the pipesare connected downwardly to the pipe lying beneath thereby serving to propel the chips in alternatingly opposite directions through the pipes, The digesting liquor containing the appropriate amounts of alkali buffer 'and sodium sulphite, and com monly referred to, as sulphited liquor, is concurrently introduced into the digester or reaction chamber at a rate of about gallons per minute.

In accordance with this invention the sulphited liquor contains an alkali metal sulphite and an alkali butter in a particular range of, proportions as follows: a mol ratio of alkali buffer (expressed as N21 CO )/alkali metal sulphite of from N05 to l/2.5. By the term alkali butter is meant principally alkali metal carbonate of whichsodium carbonateis a preferred example, although it, must be recognized that by reason of the method of determination thereof the; term also includes minor amounts of; Na S. Alkali buffer concentration is determined by first titrating for total alkali (expressed as Na O) and subtracting therefrom /a of a separate titration value for sulphite (expressed as Na O). For optimum performance utilizing the sodium carbonate and sodium sulphite it is mostpreferable that the concentration of these chemicals in the sulphited liquor be kept within the range of about 0.280 to 0.305 lb. of

3 sodium sulphite (expressed as Na O) per gallon of solution and about 0.220 to 0.238 lb. of alkali buffer (expressedas Na O) per gallon of solution.

Steam 18 under a pressure of 170 lbs. per square inch gauge is continuously and contemporaneously introduced into the digester or reactor along with the sulphited liquor. A cooking or digestion temperature of about 375 F. is thereby established. The pH within the digestion chamber is maintained at about 8.5 to by reason of the fact that the alkaline buffer neutralizes the organic acids which are formed when the wood is heated to the temperatures concerned. In accordance with this invention it is also necessary to control quite closely the relative feed rates of the wood and the sulphited liquor. This relationship is conveniently expressed in terms of the total chemical per rate of wood supplied calculated on an oven dry basis. Thus it has been found that it is most preferable that the sum of alkali bufiier and alkali metal sulphite expressed as Na O fall Within the range of from about 4.5 to of the wood calculated on an oven dry basis. This proportion of chemical to wood has been found to result in a hard high yield pulp that is very suitable for corrugating medium manufacture.

In the embodiment of this invention described in connection with the drawing, the sulphited liquor being fed at a rate of about 80 gallons per minute contained sodium sulphite in a concentration of 0.294 lb. per gallon of solution and a concentration of sodium carbonate of 0.233 lb. per gallon of solution. In accordance with these concentrations the mol ratio of alkali bufier/ sodium sulphite was l/ 1.5 and the total weight of chemical was about 9.5% of the oven dried wood.

With the above conditions a cooking residence time within the digester reactor of about 6 minutes is ample to effect chemical degradation of the wood. It may be appreciated, however, that shorter or longer residence times, or cooking times, may be employed depending upon the temperature, the concentration of the chemicals in the liquor, and the relative feed rates of the wood and the pulping liquor. A residence time substantially greater than 6 minutes is not desirable, where a high chemical charge is employed, inasmuch as such a longer residence or digestion time will result in a greater amount of the lignous component of the wood being removed and thereby decreasing the yield of pulp.

From the digester 17 the chemically attacked wood pulp passes to a defibrator 19 which exerts a mechanical shear on the chemically attacked wood chips and effects mechanical separation of the fibres. From the defibrator the pulp passes to a blow tank 20 where the pulp is diluted from about 35% consistency (solids) to about 4.5% consistency. From the blow tank 20 the pulp passes to a disc refiner 21.

The 4% consistency pulp solution leaving the latter mentioned refiner is then conveyed to a washer 22. In the washers fresh water is fed countercurrent to the pulp cleansing it of the chemicals and the lignin complexes. The washed pulp measuring about 20% consistency is screened off at the end of the washer and passes therefrom to conventional pulp processing equipment such as a series of Jordan refiners 23, storage tanks 24, and Fourdrinier paper making machines etc.

In the operation described it is found that pulp is produced at about the rate of 30,000 lbs. per hour on an air dry basis.

The filtrate 26 from the pulp washer 22 is passed to a temporary holdup tank 27 from which a small amount 28 may be conveniently fed back to the blow tank in order to form the dilute pulp of 4% consistency as described hereinbefore. This filtrate from the pulp washer is commonly referred to as black or spent liquor. Customarily this black liquor as an about 10% solids solution is utilized in conjunction with a kraft or sulphate process in order to achieve a certain economy of operation by a cross recovery. In the processes described wood. The 45-50% solids filtrate is further concentrated to about 55% solids in a cascade evaporator 29. The 55% solids solution leaving the last mentioned evaporator is pumped at about 40 to 50 gallons per minute to a recovery furnace 30. Fuel oil 31 is also fed to the furnace as a source of ignition. A portion of the solids fed to the furnace are combustible and consequently burn furnishing some heat which is recovered by means of a boiler situated within the furnace. The primary combustibles are probably the lignin sulphite orgam'c complexes formed in the digester and washed out in the filtrate in the pulp washing operation. These combustion products and the water which has been converted to steam proceed out the stack of the furnace while the noncombustibles, referred to as smelt in the molten -orrn, are allowed to flow out the bottom of the furnace into a smelt dissolving tank 32 containing water. The smelt substantially dissolves in the water forming socalled green liquor consisting primarily of sodium compounds e.g. sodium carbonate together with lesser amounts of sodium sulphide. A small amount of this smelt fails to go into solution and such is removed as dregs in a washing operation after which the dregs are directed to a waste disposal.

The green liquor is pumped to a storage tank 34 at about gallons per minute. A separate sodium carbonate tank 35 containing dissolved soda ash supplies makeup sodium carbonate solution to the green liquor storage tank as needed; usually at an average rate of about 40 to 60 lbs. of sodium carbonate per ton of air dried pulp produced. Green liquor is pumped from the green liquor storage tank at about 80 gallons per minute and is passed into a sulphiting tank 35' which is essentially a large Raschig ring packed absorption tower into which is introduced concurrently therewith sulphur dioxide. The source of the sulphur dioxide is a supply of sulphur 38 which is melted in a sulphur melter 37 and burned in a sulphur furnace 36. The sulphur re quired in accordance with the improved process which constitutes this invention amounts to about 1,000 lbs. per hour. This may also be conveniently expressed as a value of about 60 to 70 lbs. of sulphur per ton of air dried pulp produced. p

Utilizing a conventional ratio of carbonate to sulphite, as, for example, one carbonate to three sulphite, the sulphur requirement is about twice on the basis of the air dried pulp. Thus instead of about 65 to 70 lbs. of sulphur per ton of air dried pulp it is necessary to chargev about to lbs. of sulphur per ton of air dried pulp produced.

In the green liquor sulphiting operation described above, the sulphur dioxide from the sulphur burners is passed through an absorption column with the green liquor whereby the sulphur dioxide reacts with the so dium carbonate forming thereby sodium sulphite and of course some carbon dioxide. Inasmuch as some sodium sulphide is also present in the green liquor there will also be some hydrogen sulphide by-product. The latter together with the carbon dioxide escape as a gas.

The sulphited liquor is then passed to a trimming tank 39 where a final adjustment of the sodium sulphite ratio and a final adjustment of the concentration of the sulphited liquor may be made. The sulphited liquor is then stored in a holdup tank 40 ready for withdrawal on demand for introduction into the digester at the aforesaid rate of about 80 gallons per minute.

A representative sample of paper produced from pulp resulting from the described process was given a series of conventional tests in order to determine whether ornot the establishments of the ratios concerned adversely affected the properties of the paper. It was found that the said paper had a Mullen value of 42.68 lbs. and a Concora crush value of 75.49 lbs. It was further found that the paper had a tensile and tear strength, respectively, taken in the direction of the traveled paper on the Fourdrinier machine of 39.8 lbs. per inch of width, and 62.28 grams, while the same tests taken in a paper direction transverse to the direction of travel gave results of 20.78 lbs. tensile per inch of width, and a tear strength of 78.55 grams.

Contrary to what might be expected, paper produced from pulp representing a process utilizing conventional ratios of butter to sulphite did not give better results on any of the tests, thus demonstrating that there has been no deleterious eflects on the quality of paper produced by the process of this invention.

Pulp yields of about 80% or better are achievable by the process of this invention and, as can be seen, the process is fully continuous including recovery and recycle of spent liquor requiring only the addition of makeup chemicals, namely, sodium carbonate in the form of soda ash and sodium sulphite in the form of sulphur dioxide as formed from the oxidation or burning of elemental sulphur.

By means of the continuous processes disclosed herein it is possible to change from 100% capacity down to as low as 30% by making only a few minor adjustments, as, for instance, the feed rate of the chips and the pumping rate of the liquor. These adjustments can be accomplished in a very short time and without changing the character of the pulp produced therefrom or without any sacrifice in the quality of the paper produced from said pulp. This flexibility of operation, of course, makes it quite easy to control the process and permits a product of uniform characteristics.

In addition to the savings in the amount of sulphur consumed, it may be readily appreciated that a greater amount of sulphited liquor may be produced with a given size absorption tower.

It may be further noted that the use of less sulphur results in the giving ofii of a lesser amount of hydrogen sulphide gas. Obviously this may be an important consideration from the standpoint of pollution of the surrounding atmosphere.

Having thus described a particular embodiment of this invention, it will be evident to those skilled in the art that various changes and modifications may be made therein without departing from the spirit or scope of the invention as defined in the appended claims.

I claim:

1. In a continuous semi-chemical pulping process of the neutral-sulphite type for producing paper pulp from hardwood chips, said process utilizing an aqueous cooking liquor comprising an alkali metal sulphite as the primary pulping agent and an alkali metal carbonate as the primary buffering agent for neutralizing organic acids released by said wood chips upon reaction with said liquor at elevated temperatures and pressures, and wherein the aqueous filtrate from a washing step of said process is separated from the pulp, concentrated, ignited to form a ,6 smelt, and the smelt dissolved in water and sulphited to form a reconstituted liquor for reuse; the improvement which comprises introducing additional amounts of alkali metal carbonate and alkali metal sulphite into said reconstituted liquor to obtain a mo-l ratio of buffering agent consisting of alkali metal carbonate and sulphides to alkali metal sulphite ranging from about 1.0/ 0.5 to about 1.0/ 2.5, combining said reconstituted liquor with said hardwood chips in an enclosed digestion chamber, the alkali metal constituents of said liquor calculated as Na O ranging from about 4.5 to 15% by weight of wood chips calculated on an oven-dry basis, and impregnating and reacting said hardwood chips with said cooking liquor over a period not substantially in excess of about 6 minutes while maintaining a steam pressure within said chamber of about 170 psi. gauge and a temperature of about 375 F., the pH within said chamber being maintained between 8.5 and 10.0.

2. A continuous semi-chemical pulping process of the neutralsulphite type for producing pulp from hardwood chips for making paper such as corrugating medium and the like, said process comprising the steps of preparing an aqueous solution consisting essentially of alkali-metal carbonate and alkali metal sulphite by dissolving and intermixing said carbonate and said sulphite in prescribed amounts in water to form at least a portion of a reconstituted cooking liquor, the remaining major portion of said cooking liquor being derived from. a washing step of said process wherein an aqueous filtrate is separated from the pulp, concentrated, ignited to form a smelt, and the smelt dissolved in water and sulphited to form the major portion of said reconstituted cooking liquor, said reconstituted cooking liquor having a mo] ratio of buffering agent containing alkali metal carbonate and sulfides to alkali metal sulfite ranging from about 1.0/0.5 to about 1.0/ 2.5, combining said reconstituted liquor and said hardwood chips in an enclosed digestion chamber of substantial length, the alkali metal constituents of said liquor calculated as Na O ranging from about 4.5 to 15% by weight of wood chips calculated on an oven-dry basis, and impregnating and reacting said hardwood chips with said reconstituted liquor during their conveyance through said lengthy digestion chamber while maintaining a steam pressure in said chamber of about 170 psi. gauge and a temperature of about 375 F., the pH within said chamber being maintained between 8.5 and 10.0.

References Cited in the file of this patent UNITED STATES PATENTS 1,566,118 Rawling Dec. 15, 1925 1,640,853 Richter Aug. 30, 1927 1,649,281 Billwiller Nov. 15, 1927 1,859,846 Rue May 24, 1932 1,859,848 Rue May 24, 1932 1,896,038 Bradley Jan. 31, 1933 1,973,557 Bradley Sept. 11, 1934 2,022,872 Textor Dec. 3, 1953 2,774,666 Barton Dec. 18, 1956 OTHER REFERENCES Richter: (Cellulose From Hardwoods), Industrial and Engineering Chemistry, vol. 33, No. 1 (January 1941), pages to 83.

Casey: Pulp and Paper, vol. I, published by Interscience Publishers, N.Y., 1952, pages 206-212. 

1. IN A CONTINUOUS SEMI-CHEMICAL PULPING PROCESS OF THE NEUTRAL-SULPHITE TYPE FOR PRODUCING PAPER PULP FROM HARDWOOD CHIPS, SAID PROCESS UTILIZING AN AQUEOUS COOKING LIQUOR COMPRISING AN ALKALI METAL SULPHITE AS THE PRIMARY PULPING AGENT AND AN ALKALI METAL CARBONATE AS THE PRIMARY BUFFERING AGENT FOR NEUTRALIZING ORGANIC ACIDS RELEASED BY SAID WOOD CHIPS UPON REACTION WITH SAID LIQUOR AT ELEVATED TEMPERATURES AND PRESSURES, AND WHEREIN THE AQUEOUS FILTRATE FROM A WASHING STEP OF SAID PROCESS IS SEPARATED FROM THE PULP, CONCENTRATED, IGNITED TO FORM A SMELT, AND THE SMELT DISSOLVED IN WATER AND SULPHITED TO FORM A RECONSTITUTED LIQUOR FOR REUSE, THE IMPROVEMENT WHICH COMPRISES INTRODUCING ADDITIONAL AMOUNTS OF ALKALI METAL CARBONATE AND ALKALI METAL SULPHITE INTO SAID RECONSTITUTED LIQUOR TO OBTAIN A MOL RATIO OF BUFFERING AGENT CONSISTING OF ALKALI METAL CARBONATE AND SULPHIDES TO ALKALI METAL SULPHITE RANGING FROM ABOUT 1.0/0.5 TO ABOUT 1.0/2.5, COMBINING SAID RECONSTITUTED LIQUOR WITH SAID HARDWOOD CHIPS IN AN ENCLOSED DIGESTION CHAMBER, THE ALKALI METAL CONSTITUENTS OF SAID LIQUOR CALCULATED AS NA2O RANGING FROM ABOUT 4.5 TO 15% BY WEIGHT OF WOOD CHIPS CALCULATED ON AN OVEN-DRY BASIS, AND IMPREGNATING 